The present invention relates to an optical recording medium and in particular, to an optical recording medium including a recording layer made from a metal reflection film and/or recording film and a light-transmitting layer formed on a substrate so that light is introduced from the side of this light-transmitting layer for performing an information recording and/or reproduction.
A conventional optical recording, as shown in FIG. 25, has a concave-convex pattern such as a pit and a groove formed on one main surface 121a on a light-transmitting substrate 121 formed by injection molding, and on the plane of this concave-convex pattern, a metal reflection film and/or recording film (metal reflection film 122 in this case), and protection film 123 are successively formed. In such an optical recording medium 120, light is introduced via an objective lens 124 from a surface 121b of the substrate 121 not having the protection film 123, so as record and/or reproduce an information signal.
As the method for further increasing the recording density of the optical recording medium, there has been suggested a method for increasing the numerical aperture of the optical pickup objective lens so as to reduce the spot diameter of the reproduction light and to perform recording according to this.
Recently, a DVD (digital versatile disc, hereinafter, referred to as DVD) is also available in market for recording various types of data such as images, music, computer data and the like. In this DVD, the substrate has a thickness in the order of 0.6 mm to cope with a short wavelength optical system having a high numerical aperture for realizing a high-density recording.
In such circumstances, as an optical recording medium of the next generation, Japanese Patent Application 9-109660, Specification, has suggested an optical recording medium capable of four-hour recording/reproduction on one side by the NTSC (National Television System Committee) method.
This optical recording medium, enabling a civil video disc recorder to perform 4-hour recording/reproduction, has an object to have a function as a new recording medium to replace the current video tape recorder. Moreover, this optical recording medium has a configuration and size identical to that of a digital audio disc so that it can easily be handled by users who are accustomed to use digital audio discs. Furthermore, when this optical recording medium is a disc-shaped product, it is possible to utilize the high access speed which is the greatest feature of the disc shape, realizing a small-size convenient recording medium. It is also possible to provide various functions such as an instantaneous recording and reproduction as well as a trick play and edition.
The aforementioned optical recording medium need to have a recording capacity of 8 GB or more for providing such various functions.
However, in the conventional optical recording medium, especially when the size is identical to that of a digital audio disc and the information recording layer is provided only on one side, it is impossible to obtain 8-GB recording capacity. For example, in the case of DVD considered to have a high recording capacity, an information signal section area, i.e., in a region of radius of 24 to 58 mm from the center, only 4.7-GB recording capacity can be obtained, assuming the wavelength xcex to be 0.65 micrometers and the optical system numerical aperture (hereinafter, referred to as NA) to be 0.6.
For example, in order to assure a recording capacity of 8 GB or more using the DVD format as the signal format such as the ECC (error correction code) and modulation method, it is necessary to satisfy the following Expression 1.
4.7xc3x97(0.65/0.60xc3x97NA/xcex2xe2x89xa782 . . . xe2x80x83xe2x80x83(1)
According to this Expression 1, NA/lamda should be equal to or greater than 1.20. That is, it is necessary to reduce the wavelength or increase the NA.
Here, for example, if the NA is increased, it is necessary to reduce the thickness where the reproduction light transmits. This is because increase of the NA reduces the allowance of the aberration generated by the angle of the disc surface displaced from the vertical direction with respect to the optical pickup optical axis (a so-called tilt angle, proportional to the square of the product of the reverse number of the light source wavelength by the objective lens NA). This tilt angle is easily affected by the aberration due to the substrate thickness. Accordingly, the substrate thickness is reduced so as to minimize the affect from the aberration to the tilt angle.
Moreover, for the same reason, the fluctuation of thickness where the reproduction light transmits should also be within a predetermined range.
It is considered that a further higher recording density will be required in future with reduction in the substrate thickness. To cope with this, a shown in FIG. 26, there has been suggested an optical recording medium 130 including a convex-concave pattern formed on a substrate 131 whose main surface 131a is covered with a metal reflection film and/or recording film (here, metal reflection film 132), and a light-transmitting layer 133 formed successively in this order, so that light is applied from the side of this light-transmitting layer 133 for recording and/or reproducing an information signal. In this optical recording medium 130, the light-transmitting layer 133 is made thin to answer the high NA of the objective lens 134. On the other hand, in an optical recording medium, sometimes an inclination (hereinafter, referred to as a skew) of the optical recording medium with respect to the optical axis is generated to affect the signal characteristic.
This skew is usually caused by a warp of the substrate itself due to the injection molding, shrinkage of the protection film formed on the substrate, or warp of the substrate due to water absorption.
The skew present in the optical recording medium immediately after production is called initial skew. This initial skew has a configuration determined by the relationship of the force of the substrate and the force of the protection film formed. This initial skew is changed by mitigation of the stress inside the substrate and the hardening of the protection film to be stabilized finally. Furthermore, the skew due to the warp of the substrate itself and shrinkage of the protection film is changed with lapse of time for a long period of time.
On the other hand, the water absorption amount of the substrate is changed by an environmental change such as a temperature change and a humidity change. The water absorption may expand the substrate, generating a skew. Such a skew caused by an environmental change, especially the skew caused by water absorption is often seen in an optical recording medium having a signal plane only on one side, i.e. asymmetric configuration. This is because the optical recording medium having asymmetric configuration has water absorption varied over the substrate. Such a skew caused by water absorption is present in a transient condition such as water absorption and water discharge and disappears when the environment is stabilized.
The problem of the aforementioned skew in the optical recording medium, especially the skew due to water absorption is solved by waiting for half day or so. In case of a reproduction-dedicated medium, reproduction may be disabled temporarily but the medium returns to a reproduction-enabled state after waiting half day.
However, when the medium is for both of recording and reproduction, it becomes impossible to perform preferable recording and reproduction. Especially, recording cannot be performed at a desired moment and the user must miss a scene that he/she wants to record. Accordingly, in an optical recording medium capable of recording, this skew should be suppressed.
As a method to suppress the skew due to water absorption in the conventional optical recording medium 120 as shown in FIG. 25, a water-proof film can be formed on a main surface 121b of the substrate not having the protection film 123.
The water-proof film may be a metal film. In the optical recording medium 120 as shown in FIG. 25, recording and reproduction are performed by applying light from the surface 121b of the substrate not having the protection film 123. Accordingly, if a metal film as a water-proof film is formed on the surface 121b opposite to this protection film 123, it becomes impossible to assure light-transmittance for recording and reproduction because of the non-transparent metal.
Moreover, as another method for suppressing the skew due to water absorption, the substrate may be subjected to a surface treatment such as SiO so as to reduce the skew. However, in this case, a strict film formation condition should be satisfied so as to satisfy the light transmittance feature. Accordingly, there arise problems of high costs of facility, increase of production steps, lowering of yield, and the like.
Furthermore, in the optical recording medium performing recording and reproduction by applying light from the side of a light transmitting layer as shown in FIG. 26, it is possible to increase the lens NA for further increasing the recording density. However, the aforementioned skew is generated in the same way as in the conventional optical recording medium which affects the signal characteristic. There is a need for a technique for suppressing the skew.
Furthermore, it is also desired to obtain an 8-GB recording capacity or more in the optical recording medium including such a light-transmitting layer so that light is applied from the side of this light-emitting layer.
That is, in the aforementioned optical recording medium, it is necessary to suppress the skew as much as possible so as to increase the recording and/or reproduction characteristic, enabling to further increase the recording density as well as to cope with a high NA and a high capacity such as 8-GB capacity or more.
The object of the present invention is to provide an optical recording medium capable of suppressing a skew so as to enhance the recording and/or reproduction characteristic, enabling to realize a further higher recording density. Another object of the present invention is to provide an optical recording medium capable of realizing a high NA and a high capacity, enabling to record an information of 8 GB or more. Still another object of the present invention is to provide an optical disc apparatus that can preferably be used for recording and/or reproduction of an information of the optical recording medium according to the present invention.
The optical recording medium according to the present invention includes a light-transmitting layer on a main surface of a substrate, so that light is applied from the side of this light-transmitting layer for performing recording and/or reproduction. The optical recording medium has a water-proof film formed on the other main surface.
Here, in the optical recording medium according to the present invention, it is preferable that the water absorption ratio of the main surface having the water-proof film be 0.1% or below.
Moreover, in the optical recording medium according to the present invention, the aforementioned light-transmitting layer has a thickness t=3 to 177 micrometers at least in the region of an information signal section where an information signal is recorded. If xcex94t is assumed to be a thickness fluctuation of the light-transmitting film, the numerical aperture NA and the wavelength lamda of an optical system for recording and/or reproducing the optical recording medium satisfy a relationship as follows: xcex94txe2x89xa6xc2x15.26 (xcex/NA4) micrometers.
It should be noted that in the optical recording medium according to the present invention, it is preferable that the track pitch P satisfy Pxe2x89xa60.64 micrometers; and the skew "THgr"satisfy "THgr"xe2x89xa6xc2x184.115 degrees (xcex/NA3/t).
Furthermore, it is preferable that the aforementioned optical recording medium according to the present invention be recorded or reproduced by a recording/reproduction optical system that satisfies the wavelength xcexxe2x89xa60.68 micrometers and the numerical aperture NA defined as NA/xcex greater than 1.20. It is preferable to use an optical disc apparatus comprising a laser source having a wavelength equal to or below 680 nm and a lens having a numerical aperture NA equal to or above 0.7 for focusing the laser beam onto a signal recording plane.
In the optical recording medium having the aforementioned configuration according to the present invention, a water-proof film is formed on the opposite side of the substrate not having the light-transmitting layer. This enables to suppress water absorption, enabling to suppress the skew.
Moreover, in the optical recording medium according to the present invention if the water absorption ratio is suppressed to 0.1% or below, the skew can be suppressed more effectively. Here, especially the skew caused by water absorption can be suppressed effectively.
Furthermore, in the optical recording medium according to the present invention, it is further preferable that the light-transmitting layer have a thickness t defined as t=3 to 177 micrometers at least in the information signal section of the recording layer where an information signal is recorded, and a thickens fluctuation xcex94t defined as xcex94txe2x89xa6+5.26 (xcex/NA4) micrometers, assuming lamda and NA as the wavelength and the numerical aperture of the recording and/or reproducing optical system; and that the track pitch P be defined as Pxe2x89xa60.64, the skew "THgr" be defined as "THgr"xe2x89xa6xc2x184.115 degrees (xcex/NA3/t), and the wavelength lamda be defined as xcexxe2x89xa60.68; and that recording or reproduction be performed using a recording/reproduction optical system having the numerical aperture NA that satisfies the relationship NA/xcexxe2x89xa71.20. This enables to realize a high NA and a high recording capacity such as 8 GB or above. Here, such a recording or reproduction can easily be performed by using an optical disc apparatus including the aforementioned recording/reproduction optical system having a laser source of wavelength equal to or below 680 nm and a lens of numerical aperture NA equal to or above 0.7.